1. Field of the Invention
The preset invention relates to a multi-beam emitting device, and more particularly to a multi-beam emitting device which is suited to be employed in an optical switch and an optical modulator of an optical computer, an optical switch, an optical branching filter and an optical modulator of an optical communicating apparatus and an optical deflector and an optical modulator of a laser printer, a copying machine a scanner, etc.
2. Description of Related Art
A multi-beam emitting device which deflects a laser beam with a specified intensity which has been emitted from a laser source with an acoustooptic element which is driven with a plurality of electric signals with different frequencies applied thereto to split the laser beam into a plurality of laser beams has been conventionally known. When such a multi-beam emitting device is employed in an optical switch or an optical modulator of an optical computer, an optical switch or an optical branching filter of an optical communicating apparatus or the like, synchronization of a plurality of signals and simultaneous processes of a plurality of signals become possible, which is very advantageous, although the device is of a simple structure. Also, when such a multi-beam emitting device is employed in an image forming system of an electrophotographic copying machine or a laser printer, simultaneous exposure of a plurality of scanning lines becomes possible by supplying a plurality of drive signals with different frequencies, and multi-gradation image-forming becomes possible by controlling the amplitudes of the drive signals to vary the intensities (quantities of light) of the plurality of beams.
However, there have been some problems in such a conventional multi-beam emitting device having an acoustooptic element. The first problem is that the intensities of diffracted beams vary according to the number of beams to be emitted from the device, that is, that a scramble for power among beams occurs.
As a countermeasure against this problem, for example, as disclosed by Japanese Patent Laid Open Publication No. 54-83454, it has been known that a laser source 251, a waveguide type acoustooptic element 252, a signal generator 253, an amplitude control circuit 254, an image data processor 255 and a modulating circuit 256 are provided to such a multi-beam emitting device (see FIG. 21). In this device, a correction signal which depends on the number of beams to be emitted from the device is transmitted from the modulating circuit 256 to the amplitude control circuit 254, where the amplitudes of electric signals with frequencies f.sub.1, f.sub.2 and f.sub.3, respectively, generated by the signal generator 253 are controlled based on the correction signal so that the intensities of output beam L.sub.1, output beam L.sub.2, and output beam L.sub.3 can be kept constant regardless of the number of emitted beams.
As disclosed by Japanese Patent Laid Open Publication No. 54-85744, it has been also known that a laser source 261, an acoustooptic element 262, a signal generator 263, an image data processor 264, a modulating circuit 265, a photosensor 266, an intensity control circuit 267 and a make-up beam source 268 are provided to such a multi-beam emitting device (see FIG. 22). In this device, the intensity of a non-diffracted beam emergent from the acoustooptic element 262 is detected by the photosensor 266, and a detection signal of the photosensor 266 is transmitted to the intensity control circuit 267. In the intensity control circuit 267, the intensity of the non-diffracted beam is kept constant by controlling the intensity of a make-up laser beam emitted from the make-up beam source 268 so that the intensities of output beam L.sub.1, output beam L.sub.2 and output beam L.sub.3 can be kept constant regardless of the number of emitted beams.
Further, as disclosed by Japanese Patent Laid Open Publication No. 54-86360, a laser source 271, an acoustooptic element 272, a signal generator 273, an image data processor 274, a modulating circuit 275, an arithmetic circuit 276, an intensity control circuit 277 and a make-up beam source 278 are provided to such a multi-beam emitting device (see FIG. 23). In this device, data about the number of beams to be emitted from the device are transmitted from the modulating circuit 275 to the arithmetic circuit 276, and a result of calculation performed in the arithmetic circuit 276 based on the data is transmitted to the intensity control circuit 277 as a correction signal. In the intensity control circuit 277, the intensity of a make-up laser beam emitted from the make-up beam source 278 is controlled based on the correction signal so that the intensities of output beam L.sub.1, output beam L.sub.2 and output beam L.sub.3 can be kept constant regardless of the number of emitted beams.
In the multi-beam emitting device shown in FIG. 21, it is intended to alter the modulation efficiencies of the electric signals by controlling the amplitudes of the electric signals; however, there is a problem that the amplitudes and the modulation efficiencies of the electric signals do not change linearly. Further, the following problems are likely to occur: the electric signals are distorted during the amplitude control; and an unnecessary signal such as a high-frequency signal occurs during the amplitude control, thereby causing unnecessary diffracted beam by Bragg diffraction.
In the multi-beam emitting devices shown by FIGS. 22 and 23, there may be a problem that the wavelength of the make-up beam emitted from the make-up beam source 268 or 278 is not equal to the wavelength of the input laser beam emitted from the laser source 261 or 271, which causes a difference between the Bragg angle of the input laser beam and that of the make-up beam, resulting in a focal shift.
When such a multi-beam emitting device is used as an optical deflector which is employed in an image forming system of an electrophotographic copying machine or a laser printer, if a scramble for power among beams occurs, the picture quality will deteriorate. In order to prevent the scramble for power, Japanese Patent Publication No. 61-24691 has suggested that the intensities of non-diffracted light beam and regular diffracted light beam to be used for image writing are detected and that the intensity of dummy light beam is controlled so that the intensities of the non-diffracted light beam and the regular diffracted light beam can be kept constant at respective specified values.
However, according to the control method, a sensor for detecting the light intensity is necessary. Moreover, the control of the light intensity is necessary even for formation of a two-value image, and much more complicated control of the light intensity is necessary for formation of a multi-gradation image.
When such a multi-beam emitting device is used as an optical deflector which is employed in an image forming system of an electrophotographic copying machine or a laser printer, there is further another problem described below as well as the problem of a scramble for power.
An example of well-known multi-beam emitting devices is one described in Technical Report of IEICE US 92-51 (1992-09). This multi-beam emitting device is to split a laser beam into a plurality of beams with an optical waveguide type acoustooptic element which is driven with a plurality of electric signals with different frequencies applied thereto. In this device, the position of the beam waist of the laser beam is coincident with the deflecting position of the optical waveguide type acoustooptic element.
Also, Japanese Patent Publication No. 59-42855 suggested an apparatus which divides a laser beam into a plurality of beams with a crystal type acoustooptic element which is driven with a plurality of electric signals with different frequencies applied thereto and carries out simultaneous exposure of a plurality of scanning lines. In this apparatus, the deflecting position and the beam waist of the laser beam which is closer to the deflecting position are set at a specified distance from each other for the purpose of regulating the intervals among scanning lines. Meanwhile, because crystal is costly, the dimensions of the crystal type acoustooptic element are only several millimeters. Accordingly, when the deflecting position is inside the acoustooptic element, the beam waist which is closer to the deflecting position is outside the acoustooptic element.
If the multi-beam emitting device described in Technical Report of IEICE US 92-51 (1992-09) is used as a multi-beam source in a laser beam scanning optical apparatus, the following problem occurs because of the coincidence of the beam waist position and the deflecting position of the acoustooptic element with each other. When a scanning lens system is imparted with a function of correcting errors of perpendicularity of reflective facets of a polygon mirror, a plurality of beams which have been split from a laser beam by the optical waveguide type acoustooptic element are converged on a point on a scanning surface through the scanning lens system, and the intervals among scanning lines cannot be regulated.
On the other hand, with regard to the apparatus disclosed by Japanese Patent Publication No. 59-42855, because the deflecting position and the beam waist position are at a distance from each other, the apparatus does not have the problem that a plurality of beams which have been split from a laser beam by the crystal type acoustooptic element are converged on a point on a scanning surface. However, because the beam waist is positioned outside the crystal acoustooptic element, there are problems described below.
(1) When the positional relationships among the acoustooptic element and other optical elements change because of a change in environments (temperature), aging or the like, the positional relationship between the deflecting position and the beam waist changes. Thus, the intervals among scanning lines cannot be kept constant.
(2) When an error in driving the acoustooptic element occurs because of a change in environments, fluctuation of the electric power or the like, the angles of the beams emergent from the acoutooptic element, which has a high refractive index, to the air fluctuate, and the amount of fluctuation is magnified on the scanning surface in a direction in which the beams are aligned (sub scanning direction). Accordingly, because the beam waists are likely to shift in the direction, it is difficult to make uniform intervals among scanning lines.
(3) When the beam waist of the laser beam is positioned downstream from the deflecting position, the laser beams are emergent from the acoustooptic element at different angles, and accordingly, the emergent beams have mutually different astigmatic differences. Because of the differences in astigmatic difference among the laser beams, viewed from the optical system downward from the acoustooptic element, the positions of the light sources shift from desired positions by mutually different distances. Consequently, scanning lines on a scanning surface are uneven. When the beam waist is positioned upstream from the deflecting position, the beams emergent from the acoustooptic element have mutually equal astigmatic differences, and correction of these astigmatic differences is possible. However, an optical surface must be provided additionally for the correction; for example, the acoustooptic element itself (dielectric crystal) must be processed to have the astigmatic difference correction function, or a cylindrical lens or a prism must be provided additionally.